1. Field of the Invention
The present invention relates to an apparatus for forming a resist layer over an electroconductive substrate having a through hole part and/or a via hole part (i.e., a non-penetrating hole part).
2. Description of the Prior Art
When manufacturing a printed circuit board having a through hole part and/or a via hole part (hereinafter sometimes referred to as simply "through-hole part") in accordance with the prior art, a liquid resist may be applied by spraying, silkscreening or roller coating. Direct coating of a substrate with a liquid resist by any of these methods would fail to form an adequate resist film within throughholes, resulting in the problem that the interior of the throughholes is partially dissolved by the etching liquid during the etching process to invite defects such as disconnection.
In view of this problem, to protect the throughhole part from the etching liquid, tenting with a dry film resist or application of a liquid resist after filling the throughhole part with filling ink has come to be commonly used in recent years.
However, tenting does not permit narrowing the land width of throughholes and, moreover, the dry film resist it uses is more expensive than a liquid resist, resulting in the problem of an increased production cost. Filling with ink invites an increase in man-hours spent because it involves, after the filling of through-holes, a step to harden the filling ink, another step to remove by polishing the ink stuck somewhere other than the part of the substrate to be filled, and still another to remove the filling ink as required after the formation of a conductor circuit.
Therefore, the present inventors engaged in earnest research to develop, for the formation of a resist layer over an electroconductive substrate having a throughhole part, a simple and inexpensive apparatus which can protect the throughhole part from the etching liquid without having to use filling ink and which is unaffected by the land width of throughholes. As a result, they have completed the present invention by finding that the aforementioned problems can be solved by successfully pressing a liquid resist into throughholes and obtaining a smooth coat surface by roller applying the photoresist under backward rotation after roller applying it under forward rotation and before the photoresist dries. Coating an electroconductive substrate having a conductor circuit pattern and a through hole part and/or a via hole part with a solder resist or an inter-layer insulating material for built-up substrates results in complete filling of the through hole part and/or a via hole part with the resist. Thus, the formation of a reliable solder resist or an inter-layer insulator film is facilitated.